# std::expected<T,E>::or_else

From cppreference.com
< cpp‎ | utility‎ | expected

Primary template |  |
template< class F > constexpr auto or_else( F&& f ) &; | (1) | (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&; | (2) | (since C++23)
template< class F > constexpr auto or_else( F&& f ) &&; | (3) | (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&&; | (4) | (since C++23)
void partial specialization |  |
template< class F > constexpr auto or_else( F&& f ) &; | (5) | (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&; | (6) | (since C++23)
template< class F > constexpr auto or_else( F&& f ) &&; | (7) | (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&&; | (8) | (since C++23)

If *this contains an unexpected value, invokes f with the unexpected value of *this as the argument and returns its result. Otherwise, returns a `std::expected` object that represents an expected value.

1-4) The expected value is initialized with the expected value `val` of *this.

Given type `G` as:

1,2) std::remove_cvref_t<std::invoke_result_t<F, decltype(error())>>
3,4) std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(error()))>>
5,6) std::remove_cvref_t<std::invoke_result_t<F, decltype(error())>>
7,8) std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(error()))>>

If `G` is not a specialization of `std::expected`, or std::is_same_v<G::value_type, T> is false, the program is ill-formed.

1,2) These overloads participate in overload resolution only if std::is_constructible_v<T, decltype((`val`))> is true.
3,4) These overloads participate in overload resolution only if std::is_constructible_v<T, decltype(std::move(`val`))> is true.

### Parameters

f | - | a suitable function or Callable object that returns a `std::expected`

### Return value

Overload | Value of `has_value()`
--- | ---
true | false
(1,2) | G(std::in_place,`` `val`) | std::invoke(std::forward<F>(f), error())
(3,4) | G(std::in_place, std::move(`val`)) | std::invoke(std::forward<F>(f), std::move(error()))
(5,6) | G() | std::invoke(std::forward<F>(f), error())
(7,8) | std::invoke(std::forward<F>(f), std::move(error()))

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_expected` | `202211L` | (C++23) | Monadic functions for `std::expected`

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3938 | C++23 | the expected value was obtained by value()[1] | changed to **this
LWG 3973 | C++23 | the expected value was obtained by **this[2] | changed to `val`

1. ↑ `value()` requires `E` to be copy constructible (see LWG issue 3843), where `operator*` does not.
2. ↑ **this can trigger argument-dependent lookup.

### See also

transform_error | returns the `expected` itself if it contains an expected value; otherwise, returns an `expected` containing the transformed unexpected value (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/expected/or_else&oldid=173476"